The present embodiments relate to a method for operating a receiving device for magnetic resonance (MR) image signals of a body.
In an MR system for generating magnetic resonance tomograms, a body that is to be examined (e.g., the body of a human being, an animal, or a material specimen) is introduced into a basic magnetic field that may be generated, for example, by a superconducting magnetic coil. A plurality of gradient fields are superimposed on the basic field, and high-frequency alternating magnetic fields are emitted in order to induce a state in the body, in which the body radiates magnetic or electromagnetic response signals. The frequencies of the response signals of individual volume elements (e.g., voxels) of the body are dependent both on the material properties of the body and on the field strength of the magnetic field prevailing in the voxels. The frequencies are approximately equal to 42 megahertz per tesla. In order to differentiate between individual regions in the body, one of the gradient fields (e.g., the readout gradient field) for this reason effects a frequency encoding, by which voxels emit response signals along a frequency encoding axis. The center frequencies of the response signals may differ from one another by several hundred kilohertz, irrespective of the material properties.
The high-frequency signal mix emitted in total by the voxels of the body (e.g., MR image signals) is received by a plurality of receive coil elements (e.g., coil elements) and converted into electrical MR image signals. A coil element may include, for example, a wound wire. The electrical MR image signals of the coil elements are supplied to a signal processing entity, using which the electrical MR image signals are amplified, filtered and downmixed by mixers into a baseband frequency range. Both analog and digital processing may be performed. The downmixing or demodulation may be performed in two stages, with the MR image signals first being demodulated into an intermediate frequency range. The totality of circuit elements connected downstream of the coil elements may be, for example, the receive path.
The coil elements may be arranged as close as possible to the body that is to be examined. For this purpose, the coil elements may, for example, be attached to the body of a patient with the aid of a bandage or be installed in a table on which the patient lies during the examination. For example, the coil elements are no further than 30 cm away from the body. In one embodiment, the coil elements are arranged closer than 15 cm to the body. For each examination, the coil elements therefore have a different relative position with respect to the body that is to be examined. The orientation and strength of the gradient fields may also be freely set for each examination. The coil elements and the receive paths disposed downstream of the coil elements therefore are to be able to receive and process MR signals of every possible center frequency and bandwidth.
State-of-the-art receiving devices for MR systems (e.g., MR scanners) are equipped with ever more powerful gradient systems in order to satisfy the demand for faster image acquisition. In addition to the maximum gradient ascent rate (e.g., slew rate), where there are often already constraints due to physiological limits, the maximum gradient amplitude is the main performance parameter. During the data reception, the MR signals are to be transmitted amplitude- and phase-true using the full frequency encoding bandwidth. Even higher demands on the processing bandwidth of the receive paths may emerge during the multiband detection. In this case, a plurality of layers (e.g., generally parallel layers) of the body at a spatial distance are excited simultaneously, and the respective signals of all of the layers are detected simultaneously. The signal of each of the individual layers is separated by way of an additional frequency encoding along the layer normal. In addition to the readout gradient in the frequency encoding direction, a further gradient is therefore switched along the layer normal direction. The amplitudes of the two gradients and the spatial distance of the layers are coordinated with one another specifically such that the MR image signals of the individual layers are shifted into separate frequency bands. The required bandwidth is therefore multiplied by the number of layers read out simultaneously.
In order to allow the use of coil elements that deliver good image quality in a freely selectable position and for any desired excitation signals and readout gradients in an examination, the coil elements and the components of the downstream receive paths (e.g., the amplifiers, cables, connectors, switch matrices, filters, and analog/digital converters (A/D converters)) therefore support the maximum possible receive width at all times. This makes MR systems very complicated and resource-intensive in manufacturing terms and results in commensurately high production costs.